Yeah, but you can't really call it the same game either.
Gameplay, no. Lore, yes and no. New Vegas is better for that there than 3.
Still, I wasn't arguing if they're "true sequels" or not, I'm talking popularity. From a business standpoint, the more sales, the more money the company makes, and the more money it makes, the more "it works out well."
Fact is some types of gameplay have a larger appeal than others. For me, 3 and New Vegas is a lot more fun to play with if only for the real time (though 3 has problems), I was never a huge fan of turn based games. I find myself opening Fonline Reloaded a lot more than Fallout 1 or 2. And, honestly, I was never a huge isometric or 2D fan either, unless the games are really old. Personally, I'd prefer 3 and NV to have been third person, because I feel no sense in making a character to just go in first person the whole time. There IS a third person button for those but... eh, third person is clunky in those titles.
I'm not saying 1 and 2 aren't good titles, they certainly are, but other types of gameplay are a lot more popular depending on certain ages.